But when you already believe something, you are primed for things to confirm that belief, i.e. confirmation bias. One example of a black man being lazy? There you go, proof all black people are lazy. What about this black man over here being more successful than you? Ah, that's where this gets good. It's affirmative action that put him above you! Gubmint intervention, dagnabbit. His success just proves there's a white man paying for it. Again, biases confirmed!

Use these natural resentments to turn worker against worker, make them hate someone from their same oppressed class more than they hate the oppressor. Mission accomplished.